Many prior art angle transducers generate AC signals whose phase difference corresponds to the input angle. See, for example, U.S. Pat. Nos. 2,930,033 and 3,278,928. The accuracy of these devices depends in part upon the mechanical accuracy through which the signal generating elements interact to produce their signals, as well as upon the accuracy of the means used to measure the resulting phase. Several well known structures of this type include one or more pluralities of rotating poles either optically, capacitively or inductively coupled to corresponding fixed and moveable sensors. Another structure is similar, except that the mechanical roles of the poles and sensors are interchanged. It is widely held that the accuracy of such transducers begins with the generation of signals whose every cycle is a faithful translation of an input angle into a phase difference between corresponding cycles of two different signals. Averaging is frequently employed to extend the level of confidence in the measured phase. Averaging may involve simply measuring the phase over a greater number of cycles, and it may involve increasing the number of sensors and then electrically summing their outputs. But even averaging does not necessarily provide exact cancellation of pole misplacement, and unless specifically provided for in the phase measurement algorithm, variations in signal period caused by angular misplacement of the poles can produce errors in the result. In particular, the summed multiple sensor approach may introduce its own type of error if there are any variations in sensor signal amplitude. Eccentric sensor mounting, for example, can cause sensor-to-pole distance variations that in turn cause corresponding amplitude variations in the sensor signal.
A commonly employed technique to reduce a certain type of eccentricity error may actually increase the extent to which these prior art angle transducers rely upon accurate mechanical placement of their poles. This technique involves the analog summation of signals produced by one or more pairs of diametrically opposed sensors. During the summation opposing phase errors substantially cancel each other by continuously adding to almost zero. In essence, this technique combines two or more signals into a unified combination which is then used as one of two components in a phase measurement. The desired effect to be produced by the diametricity of the opposing sensors is offset or negated if the poles are significantly misplaced or the sensors are not truly diametric. That is, unless the error components to be averaged by the instantaneous summation are essentially coincident and of equal periods the desired error cancellation will not occur. This strengthens the need for the poles to be regularly placed, and adds a similarity of pole shape requirement as well.
It would be desirable if the accuracy of a shifted-signal-phase angle transducer did not fundamentally depend in part upon the accuracy of pole placement, but instead upon only the accuracy of the phase measuring means. It would also be desirable if the technique for reducing eccentricity errors did not depend upon the accuracy of pole placement or upon the symmetry of their shapes.
To continue briefly with the topic of eccentricity error correction, the error component being cancelled is a phase error. In principle, such instantaneous cancellation is nearly exact provided that the signal amplitudes from the diametrically opposed sensors are equal. Unfortunately, the nature of the eccentricity error also introduces an amplitude difference between the two signals. It would be desirable if the cancellation of the phase errors could be achieved regardless of the difference in amplitudes. These remarks apply to certain other types of errors, as well.
Variations in the angular velocity of the rotating elements can seriously affect the accuracy of the phase measuring means. Any such variations cause changes in the periods of the signals whose phase difference is to be determined. It is very desirable that the phase measuring means be essentially insensitive to both steady state changes in the angular velocity and any periodic variations that may occur within each revolution of the rotating member. Insensitivity to periodic variations reduces the need for mass to produce angular momentum ("flywheel effect") to smooth those variations, and therefore allows a lighter weight transducer.
Crosstalk between the signals for the fixed and moveable sensors can introduce phase distortion that seriously degrades the accuracy of the angle transducer. While such crosstalk can often be reduced or eliminated by shielding, that adds to the cost and mechanical complexity, adds weight, and perhaps even increases the size. It would be extremely desirable if the technique used to generate the AC signals were such that the nature of those AC signals permitted them to accurately convey their information despite the presence of crosstalk, and if the phase measurement technique were substantially insensitive to crosstalk, so that the true phase information is accurately extracted.
Prior art phase measurement techniques employed with transducers of whatever sort that generate signals of variable phase difference often produce a result best characterized as a "fine" measurement. This fine result is a modulo answer that must be combined with the result of a "coarse" measurement. How this is accomplished generally determines if the device is "incremental" or "absolute." Examples of devices to which these remarks apply include certain angle transducers and certain distance measuring equipment. While there is nothing inherently wrong with a "coarse-fine" measurement architecture, it would certainly be advantageous if the phase measurement technique employed provided a unified high accuracy and high resolution answer directly, without the need to avoid certain well known pitfalls in the averaging of separate coarse and fine components and then combining them. Some of these problems stem from the modulo nature of the measurements, and concern how to handle very small or very large values; that is, ones that are close to the value where a measurement "rolls over." While these problems have all been successfully dealt with in the past, their solutions are not cost free. It would therefore be desirable if these concerns could be dispensed with while retaining all of the accuracy and resolution of such "coarse-fine" systems. Such a phase measurement technique should also retain its insensitivity to variations in signal period (pole placement errors, motor speed variations) and to crosstalk.
An important consideration of any phase measurement technique is freedom from the so-called "phase coincidence problem." This is a problem universally experienced by what might be called the "start-stop" method of phase measurement. In this method the phase between two signals of the same frequency and of known period is found by starting a timer on a zero crossing or edge of one signal and stopping it on the corresponding zero crossing or edge of the other signal. The measured time is a fraction of the period, and thus represents the phase. A common technique of averaging with this method is to simply accumulate n-many measured intervals and divide the result by n-many periods. But this technique has a serious difficulty, especially when used with such averaging, whenever the start and stop conditions get very close together. Noise can then cause them to appear interchanged, which makes it very difficult to distinguish between and then average very large and very small angles. The usual cure for this is to introduce and then later remove a 180 degree offset in the measured values whenever they would ordinarily be within a selected region either side of zero. It would be desirable to dispense with such extra overhead while retaining the advantages of averaging.
In devices involving a rotating member absolute coarse or other information is often derived from signals indicating the completion of each revolution. It would be desirable if the generation of these once-per-revolution signals did not require extra poles or sensors.
And finally, it would be advantageous if all of the preceding benefits could be achieved in a digital system, minimizing the need for precision low-drift analog circuitry. Specifically, it is desirable to exploit the computational and decision making abilities of microprocessors to transfer the bulk of the logical complexity of the overall transducer to the processing algorithms, in conjunction with the use of suitable structural features in the measurement hardware.
These and other advantages can be realized through use of the teachings summarized below. The result is a relatively low-cost precision angle transducer requiring few precision mechanical parts but that is capable of excellent performance in the arc second range.
The angle transducer to be described achieves freedom from the need for high precision in the placement of its rotating poles by employing a phase measurement technique that is, in principle, insensitive to the effects of misplaced poles. The technique is also essentially insensitive to unequal or varying sensor-to-pole spacing. In the angle transducer to be described the rotating poles are formed by using two standard commercially available steel gears journalled upon a common shaft and driven by a motor. Fixed and moveable pairs of diametrically opposed independent (i.e., separate, and not analog summed) magnetic sensors produce four AC signals as the gears revolve.
The correction of eccentricity errors, as well as of similar errors, that is provided by diametrically opposed and independent sensors is nearly exact, and yet does not introduce a dependence upon regular or accurate spacing of the gear teeth, or a dependence upon exact diametricity. The various independent sensors each produce their own individual signals, and at least one revolution's worth of transition information contained therein is captured by periodic sampling and stored in a memory. When a measurement is to be made the transition information for each sensor is collected into an aggregate quantity and then combined with similar aggregate quantities for the other sensors. In this way all of the self-cancelling phase information is present, and does cancel when the aggregates are combined. However, the phase errors due to eccentricity need not originally be sensed in simultaneous opposition, as when the analog sensor signals are summed in real time. It is that simultaneity of opposing errors through symmetry of diametric sensors which requires ideal pole placement for maximum error cancellation. By processing the stored phase information of the signals for exactly one revolution or an integral number of exact revolutions the essential property of error symmetry at the diametric sensors is preserved, but the need for simultaneity is removed. Thus, the widths of the poles need not subtend equal angles about the axis of rotation, nor need they be placed at regular angles around that axis.
Also, since the measurement for the various sensors are made during the same revolution, various other errors that are susceptible of self cancellation, but that might not be the same from revolution to revolution, are free to reduce themselves to the maximum extent. An example of this is a ball bearing with a large ball therein.
The aggregate quantities mentioned above are formed in the process of making a phase measurement between the signals of two independent sensors. A plurality of different phase measurements are performed; one between each of certain combinations of the sensors. Specifically, the phase is measured for each combination of each moveable sensor taken in turn with each of the fixed sensors. These phase measurements are unaffected by the amplitudes of the signals involved. Once all of the various phases are in hand they may be averaged to cancel the phase error introduced by the eccentricity. In short, measure phase first and then average phase only, rather than average both phase and amplitude as inseparable entities (so that amplitude differences affect the phase difference) and then measure the phase. The cancellation of the phase error due to eccentricity is therefore nearly exact, without regard for the concomitant amplitude variations also introduced by the eccentricity or unequal pole-to-sensor distances.
Crosstalk does not substantially affect the angle transducer to be described because the signals whose phases are to be measured have different frequencies. That is, the phase information conveyed by the signals from the moveable sensors is orthogonal to the phase information conveyed by the signals from the fixed sensors. With properly chosen frequencies the integrated crosstalk result of each frequency upon the other, in principle, sums to zero. In practice, the discretely sampled nature of a digital system only approximates complete cancellation, but the approximation can, in principle, be as close as desired. The different frequencies are chosen so that neither is an integral multiple of the other. The angle transducer to be described produces signals of such frequencies by the simple expedient of using gears with different numbers of teeth.
The phase measurement technique to be described accepts signals of differing and possibly nonconstant frequencies, subject only to the following requirements. First, for P-many cycles of one frequency there must always be exactly Q-many cycles of the other. Second, some means must be employed to recurringly identify or track an absolute reference location for one or both of the signals. If both signals have absolute reference marks then an absolute (i.e., nonincremental) unified (i.e., no separate coarse and fine) result may be obtained. A coarse/fine partition of the result is also possible. If only one absolute reference mark is maintained then the result will be a fine measurement; coarse information would come either from a separate absolute measurement, or be incrementally accumulated. In accordance with certain details to be discussed later, the absolute reference marks can either be "hard" (actually part of the signal) or "soft" (the microprocessor picks certain cycles to be the marks and incrementally keeps track of how they shift relative to each other). In the former case a certain constant useful to the phase measurement technique can either be found once and permanently encoded for use by the microprocessor, or the microprocessor can automatically find and save it each time the transducer is powered up. In the latter case permanent storage of the constant may not be possible, as its value may depend upon which cycles are selected as the reference marks. Automatic constant finding in the latter case may require the operator to input one or two known static conditions to the transducer to enable the value of the constant to be discovered. In either case there is also a way to avoid having to find the value of the constant.
In the angle transducer to be described the absolute reference marks are easily generated by the simple expedient of removing an arbitrarily chosen tooth from each gear. The microprocessor recognizes the corresponding periodic disturbance in each sensor signal. This recognition locates the relative positions of the absolute reference marks. Once they are located the microprocessor can substitute an excellent approximation of what each sensor signal would have been like if those teeth had been there. This minimizes whatever effect that the missing teeth might otherwise have upon the phase measurement technique itself (no such effects are currently known) and upon the associated error reduction schemes (certain of these are known, and they tend to be second order effects).
The phase measurement itself is performed beginning at any arbitrary time during one of the P-many or Q-many cycles of the two signals. The microprocessor notes for each signal, in units of whole cycles, the differences between the starting time and the most recent occurrences of their respective absolute reference marks. Beginning with the next zero crossing of one of the signals as a local reference time, that local reference time and the zero crossing times for P-many and Q-many consecutive cycles of the respective signals are measured and stored in a table. In the embodiment to be described only positive going zero crossings are considered. Negative going zero crossings could be used instead, and a system could as easily use each zero crossing. Using the data in the table sums are formed for the P transition times in one signal and the Q transition times in the other. These sums are arithmetically combined with a measurement of the time required for P or Q cycles, the starting-time-to-absolute-reference differences, and with the values of P and Q, to produce the phase.
The present technique is free of the "phase coincidence problem," described earlier, since all that it requires is the independent measurement of P-many and Q-many consecutive times, each relative to a single reference time. Noise introduces only the unavoidable uncertainty inescapably associated with knowing those values exactly, but which uncertainty is reduced by the averaging inherent in the technique. But such noise has no further opportunity to introduce measurements that are in error by the amount of the modulus, since there is no particular correspondence between the P-many and the Q-many times. This is consistent with the advantage that pole placement can be arbitrary. What does matter is the change in the difference between the average times of occurrence for what we shall call "single equivalent poles." But each of these average times is determined separately from the other, so that the problem of phase coincidence simply cannot arise.
The above-described measurements and calculations are performed by coupling zero crossing detectors to the outputs of the independent sensors. A delay mechanism produces delayed versions of the output of each zero crossing detector. A transition detection circuit detects any transition in any of the signals by comparing delayed and undelayed values of each signal. Upon the detection of a transition the nature of the transition is captured, along with the time value in a digital clock circuit. The sequent transition and time data are temporarily stored in a revolving circular buffer to which data can be added independently from its removal. This allows short bursts of asynchronously occurring data to be captured while the microprocessor independently removes it to a memory at its own pace, under interrupt control. An up-down counter circuit indicates whether or not the revolving circular buffer contains fresh information; if it does an interrupt is generated for the microprocessor. Under the control of an interrupt service routine the microprocessor continuously updates the transition and time information stored in a table in Read/Write memory. That table contains at least one revolution's worth of data. When a request is made to perform an angle measurement the microprocessor uses the table to perform the various phase measurements between the sensors, and combines the results into a suitable answer.